Method of soft-starting a switching power supply containing phase-control clipping circuit

ABSTRACT

A method of operating a power controller that converts a line voltage to an RMS load voltage includes a soft start in which a conduction angle of a phase-control clipping circuit is gradually increased to a steady state value that defines the RMS load voltage. The controller has a circuit with a time-based signal source that triggers conduction independently of line voltage magnitude and a transistor switch whose gate receives signals from the time-based signal source to trigger the circuit. When starting the power controller, a microcontroller increases the conduction angle of the phase-control clipping circuit in plural steps to provide the soft start.

BACKGROUND OF THE INVENTION

The present invention is directed to a method of operating a powercontroller that supplies a specified power to a load, and moreparticularly to a method of starting a voltage converter that convertsline voltage to a suitable RMS load voltage, and to a lamp with asoft-start power supply.

Some loads, such as lamps, operate at a voltage lower than a line (ormains) voltage of, for example, 120V or 220V, and for such loads avoltage converter that converts line voltage to a lower operatingvoltage must be provided. The power supplied to the load may becontrolled with a phase-control clipping circuit that typically includesan RC circuit. Moreover, some loads operate most efficiently when thepower is constant (or substantially so). However, line voltagevariations are magnified by these phase-control clipping circuits due totheir inherent properties (as will be explained below) and thephase-control clipping circuit is desirably modified to provide a (morenearly) constant RMS load voltage.

A simple four-component RC phase-control clipping circuit demonstrates aproblem of conventional phase-control clipping circuits. Thephase-controlled clipping circuit shown in FIG. 1 has a capacitor 22, adiac 24, a triac 26 that is triggered by the diac 24, and resistor 28.The resistor 28 may be a potentiometer that sets a resistance in thecircuit to control a phase at which the triac 26 fires.

In operation, a clipping circuit such as shown in FIG. 1 has two states.In the first state the diac 24 and triac 26 operate in the cutoff regionwhere virtually no current flows. Since the diac and triac function asopen circuits in this state, the result is an RC series network such asillustrated in FIG. 2. Due to the nature of such an RC series network,the voltage across the capacitor 22 leads the line voltage by a phaseangle that is determined by the resistance and capacitance in the RCseries network. The magnitude of the capacitor voltage V_(C) is alsodependent on these values.

The voltage across the diac 24 is analogous to the voltage drop acrossthe capacitor 22 and thus the diac will fire once breakover voltageV_(BO) is achieved across the capacitor. The triac 26 fires when thediac 24 fires. Once the diac has triggered the triac, the triac willcontinue to operate in saturation until the diac voltage approacheszero. That is, the triac will continue to conduct until the line voltagenears zero crossing. The virtual short circuit provided by the triacbecomes the second state of the clipping circuit as illustrated in FIG.3.

Triggering of the triac 26 in the clipping circuit is forwardphase-controlled by the RC series network and the leading portion of theline voltage waveform is clipped until triggering occurs as illustratedin FIGS. 4–5. A load attached to the clipping circuit experiences thisclipping in both voltage and current due to the relatively largeresistance in the clipping circuit.

Accordingly, the RMS load voltage and current are determined by theresistance and capacitance values in the clipping circuit since thephase at which the clipping occurs is determined by the RC seriesnetwork and since the RMS voltage and current depend on how much energyis removed by the clipping.

With reference to FIG. 6, clipping is characterized by a conductionangle a and a delay angle θ. The conduction angle is the phase betweenthe point on the load voltage/current waveforms where the triac beginsconducting and the point on the load voltage/current waveform where thetriac stops conducting. Conversely, the delay angle is the phase delaybetween the leading line voltage zero crossing and the point where thetriac begins conducting.

Define V_(irrms) as RMS line voltage, V_(orms) as RMS load voltage, T asperiod, and ω as angular frequency (rad) with ω=2πf.

Line voltage may vary from location to location up to about 10% and thisvariation can cause a harmful variation in RMS load voltage in the load(e.g., a lamp). For example, if line voltage were above the standard forwhich the voltage conversion circuit was designed, the triac 26 maytrigger early thereby increasing RMS load voltage. In a halogenincandescent lamp, it is particularly desirable to have an RMS loadvoltage that is nearly constant.

Changes in the line voltage are exaggerated at the load due to avariable conduction angle, and conduction angle is dependent on the rateat which the capacitor voltage reaches the breakover voltage of thediac. For fixed values of frequency, resistance and capacitance, thecapacitor voltage phase angle (θ_(C)) is a constant defined by θ_(C)=arctan(−ωRC). Therefore, the phase of V_(C) is independent of the linevoltage magnitude. However, the rate at which V_(C) reaches V_(BO) is afunction of V_(irrms) and is not independent of the line voltagemagnitude.

FIG. 7 depicts two possible sets of line voltage V_(i) and capacitorvoltage V_(C). As may be seen therein, the rate at which V_(C) reachesV_(BO) varies depending on V_(irrms). For RC phase-control clippingcircuits the point at which V_(C)=V_(BO) is of concern because this isthe point at which diac/triac triggering occurs. As V_(irrms) increases,V_(C) reaches V_(BO) earlier in the cycle leading to an increase inconduction angle (α₂>α₁), and as V_(irrms) decreases, V_(C) reachesV_(BO) later in the cycle leading to a decrease in conduction angle(α₂<α₁).

Changes in V_(irrms) leading to exaggerated or disproportional changesin V_(orrms) are a direct result of the relationship between conductionangle and line voltage magnitude. As V_(irrms) increases, V_(orrms)increases due to both the increase in peak voltage and the increase inconduction angle, and as V_(irrms) decreases, V_(orrms) decreases due toboth the decrease in peak voltage and the decrease in conduction angle.Thus, load voltage is influenced twice, once by a change in peak voltageand once by a change in conduction angle, resulting in unstable RMS loadvoltage conversion for the simple phase-control clipping circuit.

When the phase-control power controller is used in a voltage converterof a lamp, the voltage converter may be provided in a fixture to whichthe lamp is connected or within the lamp itself. U.S. Pat. No. 3,869,631is an example of the latter, in which a diode is provided in the lampbase for clipping the line voltage to reduce RMS load voltage at thelight emitting element. U.S. Pat. No. 6,445,133 is another example ofthe latter, in which transformer circuits are provided in the lamp basefor reducing the load voltage at the light emitting element.

Each of these devices is a power controller that converts the linevoltage to an RMS load voltage and that includes a circuit that clips(in references '804, '801, and '826) or modulates (in reference '802)the load voltage to provide the RMS load voltage. The amount of clippingor modulation in the circuit is defined by a time-based signal sourcethat triggers conduction in the circuit independently of line voltagemagnitude. The circuit includes a transistor switch whose gate receivessignals from the time-based signal source to trigger operation of thecircuit. The power controller may be in a voltage conversion circuitthat converts a line voltage at a lamp terminal to the RMS load voltageusable by a light emitting element of the lamp.

The present inventors have found that these power controllers, whichhave a time-based pulse source that triggers conduction in the circuitindependently of line voltage magnitude, offer opportunities for yetfurther improvements in lamps and other devices that use these powercontrollers. The present invention seeks to take advantage of theseopportunities.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a novelmethod of operating a power controller that converts a line voltage toan RMS load voltage independently of variations in line voltagemagnitude.

A further object is to provide a novel method of soft-starting a powercontroller that uses a time-based pulse source to trigger a clipping ormodulation circuit by gradually increasing a conduction angle in theclipping circuit or gradually increasing a duty cycle in the modulationcircuit.

A still further object is to provide a lamp with this soft-start powercontroller in a voltage conversion circuit that converts a line voltageat a lamp terminal to the RMS load voltage usable by a light emittingelement of the lamp.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic circuit diagram of a phase-controlled clippingcircuit of the prior art.

FIG. 2 is a schematic circuit diagram of the phase-controlled dimmingcircuit of FIG. 1 showing an effective state in which the triac is notyet triggered.

FIG. 3 is a schematic circuit diagram of the phase-controlled dimmingcircuit of FIG. 1 showing an effective state in which the triac has beentriggered.

FIG. 4 is a graph illustrating current clipping in the phase-controlleddimming circuit of FIG. 1.

FIG. 5 is a graph illustrating voltage clipping in the phase-controlleddimming circuit of FIG. 1.

FIG. 6 is a graph showing the conduction angle α.

FIG. 7 is a graph showing how changes in the magnitude of the linevoltage affect the rate at which capacitor voltage reaches the diacbreakover voltage.

FIG. 8 is a partial cross section of an embodiment of a lamp of thepresent invention.

FIG. 9 is a schematic circuit diagram showing a first embodiment of thepower controller of the present invention suitable for forward clipping.

FIG. 10 is a schematic circuit diagram showing a second embodiment ofthe power controller of the present invention also suitable for forwardclipping.

FIG. 11 is a schematic circuit diagram showing a third embodiment of thepower controller of the present invention suitable for forward, reverse,and forward/reverse hybrid clipping and for pulse width modulation.

FIG. 12 is a graph depicting the gradual increase in conduction angle ina forward clipping embodiment of the present invention.

FIG. 13 is a graph depicting the gradual increase in conduction angle ina reverse clipping embodiment of the present invention.

FIG. 14 is a graph depicting the gradual increase in conduction angle ina forward/reverse hybrid clipping embodiment of the present invention.

FIG. 15 is a graph depicting the gradual increase in frequency and/orduration of the duty cycle in a pulse width modulation embodiment of thepresent invention.

FIG. 16 is a graph of V_(orms) versus V_(irms) for a conventional RCphase-control power controller designed to produce 42 V_(rms) output for120 V_(rms) input.

FIG. 17 is a graph of V_(orms) versus V_(irms) for a fixed phaseforward/reverse hybrid phase-control power controller incorporating thepresent invention and designed to produce 42 V_(rms) output for 120V_(rms) input.

DESCRIPTION OF PREFERRED EMBODIMENT

With reference to FIG. 8, a lamp 10 includes a base 12 with a lampterminal 14 that is adapted to be connected to line (mains) voltage, alight-transmitting envelope 16 attached to the base 12 and housing alight emitting element 18 (an incandescent filament in the embodiment ofFIG. 8), and a voltage conversion circuit 20 for converting a linevoltage at the lamp terminal 14 to a lower operating voltage. Thevoltage conversion circuit 20 may be within the base 12 and connectedbetween the lamp terminal 14 and the light emitting element 18. Thevoltage conversion circuit 20 may be an integrated circuit in a suitablepackage as shown schematically in FIG. 8.

While FIG. 8 shows the voltage conversion circuit 20 in a parabolicaluminized reflector (PAR) halogen lamp, the voltage conversion circuit20 may be used in any incandescent lamp when placed in series betweenthe light emitting element (e.g., filament) and a connection (e.g., lampterminal) to a line voltage. Further, the voltage conversion circuitdescribed and claimed herein finds application other than in lamps andis not limited to lamps.

With reference to FIG. 9 that illustrates an embodiment of the presentinvention, the voltage conversion circuit 20 clips or modulates the loadvoltage (as explained in the above cited applications that wereincorporated by reference) and includes line terminals 32 for a linevoltage, load terminals 34 for a load voltage, a transistor switch 38, atime-based signal source 40 that sends signals to a gate of thetransistor switch 38 to trigger conduction in the voltage conversioncircuit independently of line voltage magnitude, and a microcontroller42 that may be part of time-based signal source 40 or providedseparately and that controls the timing of the signals from time-basedsignal source 40. The embodiment of FIG. 9 shows a triac as thetransistor switch 38 and is suitable for forward phase control clipping.

The time-based signal source 40 may be any suitable signal source thatis capable of sending signals at constant time intervals to a gate ofthe transistor switch 38, including a pulse generator, a microcontrollerand a clock. The microcontroller may be any suitable device thatoperates a signal control program by which timing of the signals fromthe time-based signal source are controlled. The signals from thetime-based signal source should have a positive polarity at the gate ofthe transistor switch to provide the forward, reverse or forward/reversehybrid clipping.

A further embodiment that is also suitable for forward clipping is shownin FIG. 10 in which the transistor switch 38 is an SCR solid stateconduction device. This embodiment includes a full-wave bridge 44.

Yet a further embodiment that is suitable for forward, reverse, andforward/reverse hybrid clipping and pulse width modulation is shown inFIG. 11. This embodiment includes full-wave bridge 44 and an insulatedgate bipolar transistor (IGBT) that is the transistor switch 38.

Operation of the power controllers in FIGS. 9–11 is explained in theabove-mentioned applications that were incorporated by reference, theparticulars of which need not be repeated herein. In general, thetime-based signal source 40 generates pulses that are timed to coincidewith the conduction regions or modulation cycle of the power controller.The time-based signal source 40 sustains the pulses for the entirety ofeach period the transistor switch 38 is to be conducting. Some form ofsynchronization of the pulses with the load voltage waveform is alsonecessary (synchronization techniques being known and not the subject ofthe present application).

The present invention is suitable for each of these power controllers.When the lamp (or other load) is starting, microcontroller 42 increasesone of a rate and duration of the signals from the time-based signalsource 40 sent to the gate of the transistor switch 38 in plural stepsfrom a first value to a steady state value that defines the RMS loadvoltage. That is, the microcontroller gradually increases the conductionangle of phase-control power controllers or gradually increases the dutycycle of pulse width modulation power controllers from a value at whichthe RMS load voltage is very small to a larger value that defines thesteady state RMS load voltage in order to provide a soft start for thelamp. Later, during subsequent operation of the lamp, the time-basedsignal source 40 sends the signals at constant time intervals to thegate of the transistor switch to cause the voltage conversion circuit toprovide the RMS load voltage independently of line voltage magnitude.

In one embodiment, and as explained in the above-mentioned '804application, the voltage conversion circuit clips the load voltageimmediately following each polarity change of the load voltage (forwardclipping). In this event, the duration of each of the signals fromtime-based signal source 40 is increased when starting the powercontroller to increase a conduction angle of the circuit, such as shownin FIG. 12. The soft start is achieved by increasing the conductionangle from a starting value α₁ to the nominal value α_(nom) that definesthe desired RMS voltage, where the conduction angle increases with eachcycle or half cycle so that α₁<α₂<α₃<α₄<α₅<α₆<α_(nom).

In another embodiment, and as explained in the above-mentioned '801application, the voltage conversion circuit clips the load voltageimmediately before each polarity change of the load voltage (reverseclipping). In this event, the duration of each of the signals from thetime-based signal source 40 is increased when starting the powercontroller to increase a conduction angle of the circuit, such as shownin FIG. 13. The soft start is achieved by increasing the conductionangle from a starting value α₁ to the nominal value α_(nom) that definesthe desired RMS voltage, where the conduction angle increases with eachcycle or half cycle so that α₁<α₂<α₃<α₄<α₅<α₆<α_(nom).

In yet another embodiment, and as explained in the above mentioned '826application, the voltage conversion circuit clips the load voltage in aregion of the cycle of the load voltage that includes a peak valuebetween adjacent polarity changes of the load voltage (forward/reversehybrid clipping). In this event (which is, in effect, a composite of theabove two embodiments), the time-based signal source sends the signalsto the gate of the transistor switch to cause the transistor switch tobe ON during time periods that span from before to after polaritychanges of the load voltage and to be OFF between the time periods. Theduration of each of the signals is increased when starting the powercontroller to increase a conduction angle of the circuit, such as shownin FIG. 14. The soft start is achieved by increasing the conductionangle from a starting value α₁ to the nominal value α_(nom) that definesthe desired RMS voltage, where the conduction angle increases with eachcycle or half cycle so that α₁<α₂<α₃<α₄<α₅<α₆<α_(nom).

In a still further embodiment, and as explained in the above mentioned'802 application, the voltage conversion circuit modulates the loadvoltage at a frequency higher than that of the load voltage. In thisevent, the time-based signal source sends the signals to the gate of thetransistor switch to cause the transistor switch to be ON and OFF at themodulation frequency. The frequency and/or duration of the signals areincreased when starting the power controller to increase a duty cycle ofthe circuit, such as shown in FIG. 15 (which shows increases in bothfrequency and duration, it being understood that one or the other orboth may be increased in the present invention). The soft start isachieved by increasing the duty cycle from a starting value to thenominal value that defines the desired RMS voltage, where the duty cycleincreases with each cycle or half cycle.

Conventional RC phase-control clipping circuits are very sensitive tofluctuations in the line voltage magnitude. The present inventionprovides a power controller that operates substantially independently ofthe line voltage magnitude by incorporating time-based pulses to triggerconduction, thereby reducing variation of the conduction angle comparedto conventional RC phase-control circuits. Further, the soft-startprovided by the present invention eases the load conditions duringstart. For example, an incandescent filament may have longer life ifstarted with the soft-start provided herein.

FIGS. 16 and 17 illustrate the improvement afforded by the presentinvention. FIG. 16 shows relationship between V_(orms) and V_(irms) in aprior art RC phase-control clipping circuit, while FIG. 17 shows therelationship for the present invention. In each instance the circuit isdesigned to produce 42 V_(rms) output for a 120 V_(rms) input. Note thatthe output voltage varies considerably more in FIG. 16 than in FIG. 17.

The description above refers to use of the present invention in a lamp.The invention is not limited to lamp applications, and may be used moregenerally where resistive or inductive loads (e.g., motor control) arepresent to convert an unregulated AC line or mains voltage at aparticular frequency or in a particular frequency range to a regulatedRMS load voltage of specified value.

While embodiments of the present invention have been described in theforegoing specification and drawings, it is to be understood that thepresent invention is defined by the following claims when read in lightof the specification and drawings.

1. A method of operating a power controller that converts a line voltageto an RMS load voltage and that has a phase-control clipping circuitthat clips a load voltage to define a conduction angle that sets the RMSload voltage, the circuit including a time-based signal source thatsends signals to trigger conduction in the circuit independently of linevoltage magnitude and a transistor switch whose gate receives thesignals from the time-based signal source, the method comprising thesteps of: when starting the power controller, increasing the conductionangle of the phase-control clipping circuit, the conduction angleincreasing in plural steps from a first value to a steady state value;and during subsequent operation of the power controller, sending thesignals from the time-based signal source with the steady state value ofthe conduction angle at constant time intervals to the gate of thetransistor switch to cause the power controller to provide the RMS loadvoltage independently of line voltage magnitude.
 2. The method of claim1, wherein the clipping circuit clips the load voltage immediatelyfollowing each polarity change of the load voltage.
 3. The method ofclaim 1, wherein the clipping circuit clips the load voltage immediatelybefore each polarity change of the load voltage.
 4. The method of claim1, wherein the clipping circuit clips the load voltage in portions ofcycles of the load voltage that include a peak value between adjacentpolarity changes of the load voltage, and wherein the conduction anglespans from before to after polarity changes of the load voltage.
 5. Themethod of claim 1, wherein the clipping circuit includes amicrocontroller that sets the conduction angle by controlling timing ofthe signals from the time-based signal source and the method furthercomprises the step of a user selecting in the microcontroller at leastone of a location and range of the conduction angle.
 6. The method ofclaim 1, wherein the step of increasing the conduction angle of thephase-control clipping circuit comprises the step of increasing theconduction angle in each of plural consecutive cycles of the loadvoltage.